You can focus more on the story and less on the characters with this type of scenario. They already have the access to weapons and technology. They already have friends and colleagues that they can call on when the time comes. Unless you have a character like Bourne, who is trying to regain who he was, then the story is more important than character development.
The fish out of water scenario has long been used as a way to get around the superhero adventure. The fish out of water is an everyman that has no real training and is thrust into a situation. For example, an accountant that is in a bank when its robbed and is forced to protect himself and the other people from the robbers.
It's not something he wants to do or is even qualified to do, but he becomes the leader. These people often have natural heroic qualities and the adventure brings those qualities out. They most likely don't even know they have these qualities.
The fish out of water has a natural bend for comic relief and in order for them to take on and beat trained professional requires a little luck or perhaps a side kick.